In recent years, a wireless tag system attracts attention and more and more companies introduce the wireless tag system. In the wireless tag system, a wireless tag including, an IC chip and an antenna is attached to an article and ID information stored in a memory in the IC chip is read by a wireless tag communication device in a noncontact manner.
As work performed using the wireless tag system, there is work for searching for a specific wireless tag using the wireless tag communication device in an area where a large number of articles respectively attached with wireless tags are present.
In the beginning, the wireless tag communication device transmits a query radio wave from a transmitting section in a relatively large communication area. The wireless tag communication device reads, in a receiving section, all wireless tags present in a communication area according to response signals from the wireless tags. An interrogator is known that controls, if a plurality of wireless tags are present, a transmission power adjusting circuit to automatically reduce the communication area and repeats reading work. It is also disclosed that an effect same as an effect obtained by reducing transmission power can be obtained by reducing reception sensitivity.
However, in such an interrogator, the transmission power is reduced until only one RF tag is left in the communication area. If a user changes the direction of the interrogator or moves the interrogator in a direction different from the location of a search target RF tag because of some cause, the interrogator may be unable to read the search target RF tag and may lose track of the search target RF tag.
The communication area is different depending on a type of an RF tag in use, a material of an article attached with the RF tag, a method of attaching the RF tag, and the like. If transmission power in an initial state is extremely large with respect to the distance between the initial location of the user and the location of the search target and the communication area is too large, a large number of non-search target RF tags are read and an amount of change of the transmission power reduced until only one RF tag is left in the communication area is large. Therefore, the search is inefficient and a long time is required for the search.